Cigarette filters have conventionally been produced from longitudinally extended crimped filaments bonded to one another at their contact points by solvation bonds. The procedure for producing such filters involves producing a tow (or several thousand continuous filaments), crimping the tow, opening the tow to deregister adjacent crimps, uniformly applying a plasticizer to the opened tow, fluffing the tow, and thereafter condensing the plasticized tow to reduce its cross-sectional size until it is approximately equal to the cross-sectional size of a cigarette. The condensed mass is formed into a coherent structure, typically by wrapping paper around it and severing the wrapped tow into rods of predetermined length. The rods are thereafter cured to effect bonding between adjacent filaments at their contact points.
Because of the expense of the tow component of cigarette filters, it is desirable that the greatest amount of tow bulk be attained per unit weight of filamentary material. One method of expanding the tow in which the present invention is particularly well suited for use is described in U.S. Pat. No. 3,099,594 (the entire disclosure thereof being expressly incorporated hereinto by reference) wherein crimped continuous tow is fed into a jet supplied with high velocity gas whereby the crimps in the filaments are deregistered. More specifically, in the process of U.S. Pat. No. 3,099,594, a continuous multifilament crimped tow is withdrawn from a supply bale by means of a feed roll pair and passed by the suction of a blooming jet over a suitable plasticizer applicator into the blooming jet. In the blooming jet, the tow is subjected to an explosive expansion of compressed air so as to cause blooming or fluffing of the tow. The bloomed tow is then expelled from the jet under the influence of the expanding air flow into a filter rod-making machine whereby the tow is condensed into rod-like form as previously discussed.
The tow is also sometimes subjected to longitudinal tension upstream of the blooming jet according to the methods disclosed in, for example, U.S. Pat. Nos. 2,790,208 and 2,926,392, which are each incorporated hereinto by reference. According to the methods described in these patents, the longitudinally moving tow is generally subjected to continuous longitudinal tension in a zone established between a pair of tension rolls and a pair of retarded rolls (sometimes called "feed rolls" and "pretension rolls", respectively, in art parlance). The feed rolls are driven at a speed greater than the pretension rolls so that the tow is subjected in the zone therebetween to continuous longitudinal tension. Upon emerging on the downstream side of the feed rolls, the applied tension is suddenly released to thereby allow the crimped tow to relax. Such applied tension and then sudden relaxation assist in opening the tow prior to treatment by the blooming jet.
However, the tension applied in the tension zone by means of the feed and pretension roll pairs may not be uniform once an operator has set the tension in the zone for a given tow product. Such nonuniformity of applied tension may therefore be manifested in variability of the resulting filter rod characteristics, for example, pressure drop per unit rod weight, from one rod to another thereby leading to a nonuniform filter rod product.